The present invention relates to a method for determining the flux vector of a rotating field machine. The invention relates further to apparatus for carrying out the method as well as to its application.
A method for determining the flux vector of a rotating field machine is used in the apparatus according to German OS No. 30 26 202 for the field-oriented operation of a converter-fed rotating-field machine. In field orientation, the position of the flux vector is determined and the converter supplying the machine is controlled as a function of the position of the flux vector in such a way that the component of the stator current parallel to the flux and the stator current component orthogonal thereto can be influenced independently of each other. Via the control of the stator current component parallel to the flux (magnetizing current), a predetermined value for the magnitude of the flux can be set, while the current component orthogonal to the flux (active current) then enters linearly into the torque and can be used directly for the decoupled control of the speed or the torque.
For this field orientation, however, the knowledge of the flux position is necessary. In this connection, it is of advantage not to measure the flux directly via Hall probes, but by means of a computer model circuit of the electrical quantities. The simplest model for this is a so-called "voltage model", which determines the induced EMF by means of an EMF detector from the input voltages of the motor by subtracting the ohmic stator voltage drop and the inductive leakage voltages. The flux is then obtained as an integral of the EMF.
For describing the machine currents, machine voltages, the EMF and the flux, plane vectors can be used which are given by two defining quantities, for instance, their Cartesian or polar components relative to a stationary (i.e., stator-oriented "fixed-in-space") or rotating with the rotor axis ("rotor-oriented") or rotating with the field axis ("field-oriented") coordinate system. For the mentioned "voltage model", viewing in the stator-oriented Cartesian coordinate system is simplest, because for this purpose it is only necessary to form, for instance, in the case of a three-phase machine, from the voltages and currents of the three phases shifted 120.degree. relative to each other by means of a "3/2" coordinate converter, the corresponding Cartesian components fixed in space (designated here with the subscripts s1 and s2) of the corresponding stator current vector i and the stator voltage vector u, wherein the vector e of the EMF is then calculated by component-wise addition according to e=u-r.sup.s .multidot.i-l.sup..sigma. .multidot.di/dt, taking into consideration the stator resistance r.sup.s and the leakage inductance l.sup..sigma.. The Cartesian stator-oriented components of the flux vector .psi. are then obtained as the integral of the corresponding component of the EMF vector.
The open integrators required for this integration have a tendency to drift off and must be stabilized, for instance, via a null register inserted into a feedback line of the integrator. These null regulators thus form from the control deviation of the d-c components contained in the flux components respective feedback signals, from which the starting quantity for the subsequent integration by subtraction from the EMF components fixed in space is obtained.
However, with the null drift of the integration, also the correspondingly slow changes of the flux components are suppressed at low operating frequencies. In steady-state operation, an angle error is also generated which has an effect likewise primarily at low frequencies and leads to a disturbing misorientation if the reference values for the stator current are pre-set with field orientation. These disadvantages are counterbalanced, however, by the good dynamics of this voltage model.
It is possible, however, to determine a model value for the machine flux from the machine currents (i.e., the stator current vector i and, in the case of a synchronous machine, also the field current i.sup.e) and the measured rotor position, or, what frequently is advantageous from a measurement point of view, from the rotor speed of rotation. This "current model" electronically simulates the events occurring in the machine as far as they lead to the development of the flux. For this current model, the use of a field-oriented coordinate system is of advantage, the component parallel to the field being designated with the subscript .phi..sub.1 and the components orthogonal thereto with the subscript .phi..sub.2. The conversion from one coordinate system to another coordinate system rotated by a given angle is accomplished by the provision that the corresponding components of the vector to be transformed are fed to a so-called "vector rotator", to the angle input of which a suitable angle signal is applied, for instance, sine and cosine of the angle of rotation.
In the case of the current model, model parameters as accurate as possible must be set-in for the machine parameters, so that, for instance, changes of the rotor resistance due to the temperature lead to inaccuracies of the model flux in stationary as well as dynamic processes. For higher operating frequencies, the voltage model should heretofore be preferred, but at low frequencies, the current model leads to a better model value for the flux in spite of possible steady-state inaccuracies.
In the mentioned German OS 30 26 202, a combination of both models is therefore provided. According to the voltage model, two components of a model EMF vector associated with the voltage model are formed from the machine currents and machine voltages, from which then the corresponding components of the flux vector related to this voltage model as the reference vector .psi.* for the null control are formed. The circuit operates with stator orientation and, for forming the flux, contains an integrator for every Cartesian EMF component. It is achieved thereby that the voltage model is slaved to the current model at least with regard to its steady-state behavior, so that the good dynamics of the voltage model are retained, but the better steady-state flux determination of the current model is utilized at low frequencies.
The outputs of the integrators and the correction control represent the respective Cartesian stator-oriented components of a rotating vector; they must therefore continuously process alternating quantities, which cannot only be a disadvantage at high operating frequencies, and requires, especially if digitized, a high computing speed.
If, for instance, a phase error of 1.degree. in the flux determination must not be exceeded, the processing stages must not exhibit inertia greater than about 20 .mu.s. This may seem to be easily realizable only in analog technology with linear components. However, already vector rotators and other components which work with pulse width multiplication and themselves perform smoothing with time constants of, for instance, 400 .mu.s generate phase errors which, in the case of a highly dynamic control of the rotating field machine must be compensated by additional devices.
Since the presently available digital computers have computing speeds of several 100 .mu.s, the delays connected therewith do not seem to permit digitizing of such methods.